In a touch sensor, capacitance is generated depending on a capacitor element or an equivalent of the capacitor element formed by material or human contact. It is well known that capacitance may form between a pad and a human body when the body comes within a certain distance of the pad, even if the body does not touch the pad directly.
Capacitance also varies depending on the distance between the pad and a nearby human body. Capacitance increases as the human body comes closer to the pad, and decreases as the human body moves away from the pad.
In other words, the distance between the pad and the human body can be detected with accuracy by measuring the capacitance between the pad and the human body. As such, the measured capacitance can be used to determine whether or not a sensor is being touched. For instance, if the measured capacitance is greater than a predetermined capacitance, it may be determined that the sensor is being touched, and if the measured capacitance is not greater than the predetermined capacitance, it may be determined that the sensor is not being touched.
The predetermined capacitance may be set based on the natural deviation in initial capacitance. The natural deviation is caused by variation in an environment and may be calculated by conducting an experiment. The initial capacitance may be measured in a non-touched state of the sensor when the touch sensor is powered on.
The above capacitance measuring method may be applied to a touch sensor integrated circuit (IC) that is implemented with semiconductor materials. The touch sensor IC has been replacing mechanical switches in a variety of electronic products, such as cellular phones, television receivers, air conditioners, microwave ovens, and so on.
The capacitance generated between a pad of the touch sensor IC and a human body may range from about several picofarads to several tens of picofarads. As such, a variety of methods have been proposed to accurately calculate or measure capacitance.
As one example of a capacitance measuring method in accordance with the related art, capacitance may be measured based on a charging time of a capacitor. In such a method, the capacitor is discharged to the ground, and then charged up to a reference voltage level by using a constant current source. When the capacitor is being charged, the charging time is measured by a timer that counts high-speed clocks.
In the above capacitance measuring method, the capacitor must be discharged in a considerably short time because of the limitations of the high-speed clock of the timer. Also, the current source in this capacitance measuring method supplies a very small current so that the capacitor is charged long enough to accurately measure the capacitance.
The smaller the charging current becomes, the greater the values that can be obtained from the timer. However, if the charging current is too small, influences from external noise and a parasitic current within the touch sensor may increase. As such, the charging time of the capacitor may be increased or decreased by the noise. Thus, the capacitor may not be efficiently charged.
As another example of a capacitance measuring method of the related art, capacitance may be measured based on a discharging time of the capacitor. More specifically, the capacitor may be charged up to a supply voltage (VDD) level, and then discharged to the reference voltage level using a resistor that is connected to the ground. When the capacitor is being discharged, the discharging time is measured by a timer that counts high-speed clocks.
In this capacitance measuring method, the capacitor must be fully charged in a considerably short time due to the limitations of the high speed clock of the timer. Also, a considerably large resistor (having resistance of at least one mega-ohm) should be employed for discharging the capacitor to accurately measure the capacitance. The discharging current may range from several hundreds of pico-amperes to several tens of micro-amperes when the charged capacitor is discharged through the resistor.
As described above, the capacitance generated between the pad and the human body ranges from about several picofarads to several tens of picofarads. As such, the smaller the discharging current becomes, the greater the values that can be obtained from the timer. However, if the discharging current is very small, influences from external noise and a parasitic current within the touch sensor may increase, and the discharging time of the capacitor may be increased or decreased by the noise. Accordingly, the capacitor cannot be efficiently discharged, and it is difficult to measure the discharging time of the capacitor.
Moreover, the capacitance measuring methods of the related art cannot properly measure mutual capacitance due to charge sharing. If the capacitance of a target capacitor is accurately measured, the mutual capacitance may also be accurately measured. However, if it is difficult to accurately measure capacitance, a considerably large capacitor is used. As a result, an external capacitor is used and different weight values are set on panels to calculate the capacitance.